Interpretive Summary: Better understanding of water stress definition is needed to improve crop production simulation. In this study, two simulation models, the Root Zone Water Quality Model (RZWQM) and RZWQM-SHAW (Simultaneous Heat and Water) (RZ-SHAW) Hybrid Model, were evaluated for their ability to simulate corn and soybean growth and water use under a range of water conditions in the Central Great Plains. In both models, water stress index (WSI) was calculated based on a nonlinear form of the ratio of actual (AT) and potential transpiration (PT). Results showed that in general RZWQM using daily evapo-transpiration (ET) provided similar simulation for both corn and soybean production as RZ-SHAW using the hourly ET option, given that the same plant parameters were used. However, RZ-SHAW with SHAW ET provided the worst predictions for corn and soybean growth. The reason for the ill-behavior of SHAW ET was due to differences in AT and PT calculations as that in RZWQM, which might warrant recalibration of the plant parameters. The results also showed that, when two models were linked to develop a hybrid model, it was essential for variables to communicate correctly between components of the two models, not only in unit but also in magnitude. This study further demonstrated that, when RZ-SHAW is used for plant simulation, AT and PT should be calculated the same way as in RZWQM to preserve the plant parameters. Otherwise, recalibration of plant growth parameters is needed. This study also showed that the nonlinear form of WSI was applicable to both crops and all the ET calculations, although the alpha value varied among the two crops and various ET calculations.

Technical Abstract:
Water stress calculations in agricultural systems models are empirical and yet control all biological processes under sub-optimum water conditions. Better understanding of water stress definition is needed to improve crop production simulation. In this study, two simulation models, the Root Zone Water Quality Model (RZWQM) and RZWQM-SHAW (Simultaneous Heat and Water) (RZ-SHAW) Hybrid Model, were evaluated for their ability to simulate corn and soybean growth and water use under a range of water conditions in the Central Great Plains. In both models, water stress index (WSI) was calculated based on a nonlinear form of the ratio of actual and potential transpiration [WSI=(AT/PT)alpha, 0<alpha<1]. Potential evapotranspiration (PET) was calculated using an extended Shuttleworth-Wallace approach in RZWQM on a daily basis (ETSW-DAY) and was calculated in RZ-SHAW using either the extended Shuttleworth-Wallace approach on hourly basis (ETSW-HR) or the SHAW approach (ETSHAW). Simulated results of the three ET methods and different water stress index (WSI) calculations were compared to measured crop growth and soil water measurements from corn and soybean studies under semi-arid Colorado conditions. Results showed that in general RZWQM using ETSW-DAY provided similar simulation for both corn and soybean production as RZ-SHAW using the ETSW-HR option, given that the same plant parameters were used. However, RZ-SHAW with ETSHAW provided the worst predictions for corn and soybean growth. The reason for the ill-behavior of ETSHAW was due to differences in AT and PT calculations as that in RZWQM, which might warrant recalibration of the plant parameters. The results also showed that, when two models were linked to develop a hybrid model, it was essential for variables to communicate correctly between components of the two models, not only in unit but also in magnitude. This study further demonstrated that, when RZ-SHAW is used for plant simulation, AT and PT should be calculated the same way as in RZWQM to preserve the plant parameters. Otherwise, recalibration of plant growth parameters is needed. This study also showed that the nonlinear form of WSI was applicable to both crops and all the ET calculations, although the alpha value varied among the two crops and various ET calculations.